A “flip chip” is a semiconductor device that includes a pattern of solder balls arrayed on one of the chip's surfaces. During fabrication, the solder balls are formed on the top surface of the chip. The chip can then be readily mounted onto, for example, a circuit board by flipping the chip so that the top surface faces downward and the solder balls are aligned with corresponding pads on the circuit board.
In a non-flip chip semiconductor device, the gate and source contacts are on one surface (e.g., the top surface) of the chip while the drain contact is on the opposite surface (e.g., the bottom surface) of the chip. In a flip chip, the gate, source, and drain contacts are on the same surface of the chip. To form a circuit between the source and drain in a conventional flip chip, a “diffusion sinker” or “deep sinker” 110 is formed in the epitaxial layer 150 between the drain 120 and the substrate layer 130 as shown in FIG. 1. In operation, current will flow from the source 140 to, and through, the substrate and then to the drain in a known manner.
The sinker is formed using an isotropic diffusion process in which a dopant material is diffused through the epitaxial layer until the sinker is in contact with the substrate. However, as the sinker diffuses downward (in the y-direction), it also diffuses both along the surface of the structure (in the x-direction) and into the higher resistivity epitaxial layer. Thus, the surface area of the sinker is relatively large, increasing the size of each cell and thereby decreasing cell density. Also, the resistance of the epitaxial layer increases as the size of the sinker increases, thereby increasing the device's on-resistance (Rdson). Furthermore, the isotropic diffusion process is relatively slow, which increases the time needed to manufacture the flip chips and decreases throughput, thereby increasing costs.